![]() 1 top) show similar decrease with height, this is not the case of polar densities (Fig. ![]() For each set of observations, the time difference relative to the nearest solar maximum is indicated in years, along with the corresponding symbol.Īlthough not exhaustive, this set of models shows a dispersion of results over about one order of magnitude for both equatorial and polar regions. Figure 1 gathers the results from various data sets for the heliocentric range 1.0–1.5 R ⊙ and for quiet equatorial and polar regions.ĭensity models for quiet corona in equatorial regions ( top) and polar regions ( bottom). 1995), extreme-UV (EUV) and soft X-rays observations were also used to derive coronal densities, with the following limitations: i) they are generally limited to heliocentric distances <1.2 R ⊙ for the quiet corona near the solar equator, less in coronal holes, and must be completed at greater heliocentric distances with white light observations, now also from space observations ii) they are restricted to relatively short observational campaigns and have limited statistical significance. Since the launch of Skylab (May 1973) and, more, recently of SoHO (Dec. Conversely, ground-based coronographic observations allowed continuous time coverage but were restricted to larger heliocentric distances (typically >1.2 R ⊙). 1.05 times the solar radius ( R ⊙) but were restricted to eclipse times. Eclipse observations allowed the density to be derived down to low heliocentric distances, e.g. 1999 Meyer-Vernet 2012).īefore space observations, coronal density measurements relied only on white light observations. In particular, the variations in density with height is a key parameter when modeling the acceleration of the fast solar wind, giving some constraints on mechanical forces and local deposit of energy in the low and medial corona (Lallement et al. IntroductionĮlectron density and temperature are basic variables for describing the solar corona and the physical processes that occur there. Which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License ( ), The electron kinetic temperature T e is substantially less than T H. The radio models are generally less dense, which is compatible with isothermal hydrostatic equilibrium in their range of heliocentric distances, and they show different behaviors with the solar cycle in the equatorial or polar radial directions. ![]() The yearly-averaged variations of these models are less than the dispersion between models derived from other techniques, such as white light and EUV observations, partly because these two techniques are not time-averaged, and they refer to particular days. This implies an ion temperature T i ~ 2.2 MK.Ĭonclusions. The kinetic temperature T e of electrons in the corona ( ~0.62 MK) is found to be significantly less than T H ( ~1.5 MK). Changes in n 0 and T H with solar cycle are given for equatorial and polar regions. They are characterized by their density value n 0 extrapolated down to the base of the corona and their scale-height temperature T H. The derived yearly-averaged density models along equatorial and polar diameters are consistent with isothermal and hydrostatic models. These minima are more pronounced for EW profiles than for NS ones. The widths of the brightness profiles that were averaged yearly have minima at cycle minimum (2008–2009). The electron temperature, in turn, can be derived from the comparison of the observed mean spectra on the disk with those predicted through transfer calculations from the density models derived from limb observations. The agreement between results from different frequencies, in the ranges of r where there is overlapping shows the robustness of the method. The total ranges in the heliocentric distance r are 1.15–1.60 R ⊙ (EW) and 1.0–1.4 R ⊙ (NS). Measurements of the brightness temperature T b beyond the limb allowed coronal density models to be derived in both EW and NS radial directions, with a weak dependence on the electron temperature. Images at 6 frequencies between 150 and 450 MHz for 183 quiet days between 20 were used. We study the variations of the quiet corona in brightness and size during an 8-year period and derive electron density and temperature in the corona. The 2D images obtained through rotational aperture synthesis with the Nançay Radioheliograph are suitable for quantitative exploitation. Paris-Diderot, 92195 Meudon Cedex, FranceĬontext. LESIA-Observatoire de Paris, CNRS, UPMC, Univ. Astronomical objects: linking to databases.Including author names using non-Roman alphabets.Suggested resources for more tips on language editing in the sciences Punctuation and style concerns regarding equations, figures, tables, and footnotes
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